Sleep involves the pineal gland. Why wouldn't sleep paralysis?
Ah, OK, I see; now I think I understand where you are coming from.
Well, sleep is a very complex process that involves many different brain regions. Sleep paralysis depends critically on a set of cells in the brainstem (in the pons) that appear to use acetylcholine as a neurotransmitter.
Sleep, itself, is not one thing. There are many processes involved in it. There is sleep initiation which is governed by one area, the timing of sleep, which is governed by at least two areas -- one in the hypothalamus (the suprachiasmatic nucleus is one of them) and the pineal with the pineal being less important. In fact you can live without a pineal gland and sleep just fine. Then there is the shift between different stages of sleep, including the shift into and out of REM sleep.
Sleep paralysis (and we need to be very careful here in defining this state, since there seem to be folks using the words in different ways) refers to the continued state of complete and utter paralysis that naturally occurs during REM sleep, but which may persist beyond the REM state if the cells responsible for it continue to fire inappropriately (that is the putative explanation). Cataplexy is a somewhat similar state in which a person may lose all muscle tone for a brief time if excited while awake.
So, this is not a state of complete relaxation. It is actually a profound paralysis -- complete, except for the eyes and respiratory muscles, which can continue to move. If you were in a state of sleep paralysis, for instance, it would be impossible to maintain a seated position -- you would necessarily be on the floor or bed (or wherever) in a state utterly and completely unable to move even though wide awake.
Just in case there is any confusion for anyone reading this thread, sleep paralysis does not refer to the feeling of heaviness as you come out of sleep, or to states in which you are clearly dreaming (unless there is a definite association with hypnapompic hallucination, which happens but it not that common -- most folks with hypnopompic hallucinations feel weird and weak but don't really experience sleep paralysis with it in my experience). Sleep paralysis is a very, very frightening state which some in this thread have described quite well. I am a little sceptical that we are all talking about the same thing. I know from experience that there are many states associated with sleep and relaxation that folks call sleep paralysis.